An 84-year-old man in Ireland stunned doctors when scans revealed that he seemed to be missing a large chunk of his brain. Instead of brain tissue, the doctors found a 9cm (~3.5 inch) pressurized pocket of air where much of his right frontal lobe ought to be. The doctors reported the discovery recently inBMJ Case Reports.

The empty head space was particularly surprising because the man arrived in the emergency department with afflictions otherwise common for his age. He had been complaining to his regular doctor about repeated falls and feeling unsteady in recent months. When the man added left-sided arm and leg weakness to the list of complaints, his doctor advised him to go to the emergency room, fearing a possible stroke.

But aside from the weakness and unsteadiness, the man was in good shape. In the case report, doctors noted that “there was no confusion, facial weakness, visual or speech disturbance… He was otherwise fit and well, independent with physical activities of daily living (PADLs) and lived at home with his wife and two sons. He was a non-smoker and drank alcohol rarely."

Additionally, blood tests in the emergency department came back normal, and he showed normal eye movement ranges, visual fields, speech, and swallowing. It was only when the doctors performed CT and MRI scans that they discovered the large, pressurized air mass in his brain, called a pneumatocoele.

Lead author Finlay Brown, a physician with the Northern Ireland Medical and Dental training agency who reviewed the man’s scans, called the case “perplexing.” Usually, such air pockets are seen—on a smaller scale—in those who have undergone brain surgery. But the man had not.

On closer inspection, the doctors found that the scans revealed the likely source of the problem: a benign bone tumor in his sinus that had eroded a tiny passageway through his skull. Based on the small number of similar cases reported, the doctors speculated that the tumor had created a small opening that pushed air into the brain like a “one-way valve.”

“From speaking to the specialists, it seems it has been progressing insidiously over months to years,” Brown told the WashingtonPost. “When the patient sniffed/sneezed/coughed he would most likely be pushing small amounts of air into his head.”

The doctors also noted that brain air pockets like this have, in rare cases, been reported to cause small strokes. That could explain the small stroke that the man was found to have suffered and caused his left-side weakness.

After consulting with specialists about surgery to release the air and remove the bone tumor, the man opted to skip any such intervention, given the risks. In a 12-week follow-up, the man was reported to be doing fine and his left-side weakness had cleared up.

The doctors say the case illustrates the importance of using imaging in initial assessments of neurological symptoms—even when the symptom seem routine.